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Elucidating the initial steps in α-uranium hydriding using first-principles calculations

Hydrogen embrittlement of uranium, which arises due to the formation of a structurally weak pyrophoric hydride, poses a major safety risk in material applications. Previous experiments have shown that hydriding begins on top or near the surface (i.e., subsurface) of a-uranium. However, the fundamental molecular-level mechanism of this process remains unknown. In this work, starting from pristine α-U bulk and surfaces, we present a systematic investigation of possible mechanisms for formation of the metal hydride. Specifically, we address this problem by examining the individual steps of hydrogen embrittlement, including surface adsorption, subsurface absorption, and the inter-layer diffusion of atomic hydrogen. Furthermore, by examining these processes across different facets, we highlight the importance of both (1) hydrogen monolayer coverage and (2) applied tensile strain on hydriding kinetics. Taken together, by studying previously overlooked phenomena, this study provides foundational insights into the initial steps of this overall complex process. We anticipate that this work will guide near-term future development of multiscale kinetic models for uranium hydriding and subsequently, identify potential strategies to mitigate this undesired process.